The present invention, in some embodiments thereof, relates to apparatus and a method for sub-resolution optical detection and, more particularly, but not exclusively, to such apparatus and a method for detection in a three-dimensional space of user interactions for operating digital equipment.
In digital image processing, sub-pixel resolution can be obtained in digital images containing well defined lines, points or edges that can be processed by an algorithm to reliably measure the position of the line, point or edge in the image with an accuracy exceeding the nominal pixel resolution of that image.
Thus, for example, if the image of a car of length 50 cm, viewed side-on is 500 pixels long the nominal resolution (pixel size) on the side of the ship facing the camera is 0.1 cm. Now sub-pixel resolution of well resolved features can measure ship movements which are an order of magnitude (10×) smaller. Movement is specifically mentioned in this discussion of the existing art because measuring absolute positions requires an accurate lens model and known reference points within the image to achieve sub-pixel position accuracy. Small movements can however be measured (down to 0.1 mm) with simple calibration procedures.
Generally, digital image processing systems are limited in resolution by a number of factors. One of these is the pixel size of the detector. Another is the nature of the scene being detected and another is the quality of the optics used to focus light from the scene onto the detector.
The system designer thus has the options of improving the optics and/or using a detector with a smaller pixel size. However both of these options increase costs. In the case of trying to detect user interactions, which is a special case of image processing, the user may be several meters from the screen, and control gestures that need to be detected may involve individual fingers.
Three-dimensional detection systems often use active illumination. In one known system, an arrangement of lasers is used to illuminate the target. The use of active illumination further complicates the issue of resolution, since the laser light beam positions are correlated with the depth of the object they are reflected from as described in triangulation depth detection methods. Therefore detecting the light feature position in sub-resolution enables measuring of the depth of the scenery in higher resolution as well. The depth issue becomes even more important when the object is located at a larger distance from the sensor. Since the laser beam is usually collimated at least in one axis and each camera pixel samples the scenery in an angular fashion, the light features may be sampled by less than a single pixel therefore preventing detection of accurate position and depth.